Code review comment for lp:~mwhudson/launchpad/much-faster-rewrite-map

As we've discovered, we need the rewriting done by the branch-rewrite.py script to be much quicker. So this branch implements it via a single database query. This should (a) be much much faster (b) take load off the XML-RPC server.

In addition, it records in the log whether the cache was hit or not.

There is now some duplicated code between lp.codehosting.rewrite and lp.codehosting.vfs.branchfsclient, which could perhaps be reduced with some cleverness. OTOH, an extremely un-abstract, bare bones approach seems sane for something we've just found is so performance sensitive.